Perpendicular magnetic recording exchange-spring type medium with a lateral coupling layer for increasing intergranular exchange coupling in the lower magnetic layer

ABSTRACT

A perpendicular magnetic recording system and medium has a multilayered recording layer that includes an exchange-spring structure and a ferromagnetic lateral coupling layer (LCL). The exchange-spring structure is made up of two ferromagnetically exchange-coupled magnetic layers (MAG 1  and MAG 2 ), each with perpendicular magnetic anisotropy. MAG 1  and MAG 2  may have a coupling layer (CL) located between them that permits ferromagnetic exchange coupling of MAG 1  with MAG 2 . The LCL is located either above or below MAG 1  and in direct contact with MAG 1  and mediates an effective intergranular exchange coupling in MAG 1 . The ferromagnetic alloy in the LCL has significantly greater intergranular exchange coupling than the ferromagnetic alloy in MAG 1 , which typically will include segregants such as oxides. The LCL is preferably free of oxides or other non-metallic segregants, which would tend to reduce intergranular exchange coupling in the LCL. Because the LCL grain boundaries overlay the boundaries of the generally segregated and decoupled grains of MAG 1 , and the LCL and MAG 1  grains are strongly coupled perpendicularly, the LCL introduces an effective intergranular exchange coupling in the MAG 1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to perpendicular magnetic recordingmedia, such as perpendicular magnetic recording disks for use inmagnetic recording hard disk drives, and more particularly to aperpendicular magnetic recording medium with an “exchange-spring”recording layer structure.

2. Description of the Related Art

Horizontal or longitudinal magnetic recording media, wherein therecorded bits are oriented generally parallel to the surfaces of thedisk substrate and the planar recording layer, has been the conventionalmedia used in magnetic recording hard disk drives. Perpendicularmagnetic recording media, wherein the recorded bits are stored in therecording layer in a generally perpendicular or out-of-plane orientation(i.e., other than parallel to the surfaces of the disk substrate and therecording layer), provides a promising path toward ultra-high recordingdensities in magnetic recording hard disk drives. A common type ofperpendicular magnetic recording system is one that uses a “dual-layer”medium. This type of system is shown in FIG. 1 with a single write poletype of recording head. The dual-layer medium includes a perpendicularmagnetic data recording layer (RL) on a “soft” or relativelylow-coercivity magnetically permeable underlayer (SUL) formed on thesubstrate. The RL is typically a granular ferromagnetic cobalt alloy,such as a CoPtCr alloy with a hexagonal-close-packed (hcp) crystallinestructure having the c-axis oriented generally perpendicular to the RL.

The SUL serves as a flux return path for the field from the write poleto the return pole of the recording head. In FIG. 1, the RL isillustrated with perpendicularly recorded or magnetized regions, withadjacent regions having opposite magnetization directions, asrepresented by the arrows. The magnetic transitions between adjacentoppositely-directed magnetized regions are detectable by the readelement or head as the recorded bits.

FIG. 2 is a schematic of a cross-section of a prior art perpendicularmagnetic recording disk showing the write field H acting on therecording layer RL. The disk also includes the hard disk substrate thatprovides a generally planar surface for the subsequently depositedlayers. The generally planar layers formed on the surface of thesubstrate may also include a seed or onset layer (OL) for growth of theSUL, an exchange break layer (EBL) to break the magnetic exchangecoupling between the magnetically permeable films of the SUL and the RLand to facilitate epitaxial growth of the RL, and a protective overcoat(OC). As shown in FIG. 2, the RL is located inside the gap of the“apparent” recording head (ARH), which allows for significantly higherwrite fields compared to longitudinal or in-plane recording. The ARHcomprises the write pole (FIG. 1) which is the real write head (RWH)above the disk, and a secondary write pole (SWP) beneath the RL. The SWPis facilitated by the SUL, which is decoupled from the RL by the EBL andproduces a magnetic image of the RWH during the write process. Thiseffectively brings the RL into the gap of the ARH and allows for a largewrite field H inside the RL. However, this geometry also results in thewrite field H inside the RL being oriented nearly normal to the surfaceof the substrate and the surface of the RL, i.e., along theperpendicular easy axis of the RL grains, as shown by typical grain 1with easy axis 2. The nearly parallel alignment of the write field H andthe RL easy axis has the disadvantage that relatively high write fieldsare necessary to reverse the magnetization because minimal torque isexerted onto the grain magnetization. Also, a write-field/easy-axisalignment increases the magnetization reversal time of the RL grains, asdescribed by M. Benakli et al., IEEE Trans. MAG 37, 1564 (2001).

For these reasons, “tilted” media have been theoretically proposed, asdescribed by K.-Z. Gao et al., IEEE Trans. MAG 39, 704 (2003), in whichthe magnetic easy axis of the RL is tilted at an angle of up to about 45degrees with respect to the surface normal, so that magnetizationreversal can be accomplished with a lower write field and without anincrease in the reversal time. However, there is no known fabricationprocess to make a high-quality recording medium with a RL having atilted easy axis.

A perpendicular recording medium that emulates a tilted medium and iscompatible with conventional fabrication processes has been proposed.This type of medium uses an “exchange-spring” structure in the RL toachieve a magnetic behavior that emulates the behavior of a tiltedmedium. In an exchange-spring perpendicular recording medium, the RLstructure is a composite of a magnetically “hard” layer (highercoercivity) and a magnetically “soft” layer (lower coercivity) that areferromagnetically exchange-coupled. An intermediate coupling layer maybe located between the hard and soft magnetic layers to reduce thestrength of the interlayer exchange coupling. The two magnetic layerstypically have different anisotropy fields (H_(k)). (The anisotropyfield H_(k) of a ferromagnetic layer with uniaxial magnetic anisotropyK_(u) is the magnetic field that would need to be applied along the easyaxis to switch the magnetization direction.) In the presence of auniform write field H the magnetization of the lower-H_(k) layer willrotate first and assist in the reversal of the magnetization of thehigher-H_(k) layer, a behavior that is sometimes called the“exchange-spring” behavior. Exchange-spring perpendicular recordingmedia are described by R. H. Victora et al., “Composite Media forPerpendicular Magnetic Recording”, IEEE Trans MAG 41 (2), 537-542,February 2005; and J. P. Wang et al., “Composite media (dynamic tiltedmedia) for magnetic recording”, Appl. Phys. Lett. 86 (14) Art. No.142504, Apr. 4, 2005. Pending application Ser. No. 11/231,516, filedSep. 21, 2005 and assigned to the same assignee as this application,describes a perpendicular magnetic recording medium with anexchange-spring RL structure formed of a lower high-H_(k) ferromagneticlayer, an upper low-H_(k) ferromagnetic layer and an intermediatecoupling layer between the two ferromagnetic layers.

The problem of thermal decay exists for perpendicular recording mediawith conventional RLs and for media with exchange-spring RL structures.As the thickness of the RL structure decreases, the magnetic grainsbecome more susceptible to magnetic decay, i.e., magnetized regionsspontaneously lose their magnetization, resulting in loss of data. Thisis attributed to thermal activation of small magnetic grains (thesuperparamagnetic effect). The thermal stability of a magnetic grain isto a large extent determined by K_(u)V, where K_(u) is the magneticanisotropy constant of the layer and V is the volume of the magneticgrain. Thus a RL with a high K_(u) is important for thermal stability.However, in a medium with an exchange-spring RL structure, one of themagnetic layers has very low K_(u), so that this layer cannot contributeto the thermal stability of the RL.

To address the problem of thermal decay in exchange-spring media,pending application Ser. No. 11/372,295, filed Mar. 9, 2006 and assignedto the same assignee as this application, describes a perpendicularrecording medium with an exchange-spring RL structure formed of twoferromagnetic layers with substantially similar anisotropy fields H_(k)that are ferromagnetically exchange-coupled by an intermediatenonmagnetic or weakly ferromagnetic coupling layer. Because the writehead produces a larger magnetic field and larger field gradient at theupper portion of the RL, while the field strength decreases furtherinside the RL, the upper ferromagnetic layer can have a high anisotropyfield. The high field and field gradient near the top of the RL, wherethe upper ferromagnetic layer is located, reverses the magnetization ofthe upper ferromagnetic layer, which then assists in the magnetizationreversal of the lower ferromagnetic layer and causes the overallnon-uniform magnetization reversal that is typical for exchange-springmedia. Because both ferromagnetic layers in this exchange-spring type RLhave a high anisotropy field and are sufficiently exchange coupled, thethermal stability of the medium is not compromised.

Both horizontal and perpendicular magnetic recording media that userecording layers of granular ferromagnetic cobalt alloys exhibitincreasing intrinsic media noise with increasing linear recordingdensity. Media noise arises from irregularities in the recorded magnetictransitions and results in random shifts of the readback signal peaks.High media noise leads to a high bit error rate (BER). Thus to obtainhigher areal recording densities it is necessary to decrease theintrinsic media noise, i.e., increase the signal-to-noise ratio (SNR),of the recording media. The granular cobalt alloys in the RL structureshould thus have a well-isolated fine-grain structure to reduceintergranular exchange coupling, which is responsible for high intrinsicmedia noise. Enhancement of grain segregation in the cobalt alloy RL canbe achieved by the addition of segregants, such as oxides of Si, Ta, Ti,Nb, Cr, V, and B. These oxides tend to precipitate to the grainboundaries, and together with the elements of the cobalt alloy, formnonmagnetic intergranular material.

However, unlike horizontal recording media, where the complete absenceof intergranular exchange coupling provides the best SNR, inperpendicular recording media the best SNR is achieved at someintermediate level of intergranular exchange coupling. Also,intergranular exchange coupling improves the thermal stability of themagnetization states in the media grains. Thus in perpendicularrecording media, some level of intergranular exchange coupling isadvantageous.

Pending application Ser. No. 11/532,055 filed Sep. 14, 2006 and assignedto the same assignee as this application, describes a perpendicularmagnetic recording medium with an exchange-spring RL structure having alateral coupling layer (LCL) that is in contact with the upper magneticlayer and mediates intergranular exchange coupling in the upper magneticlayer.

What is needed is a perpendicular magnetic recording medium with anexchange-spring RL structure that has optimal intergranular exchangecoupling to produce high SNR, and high thermal stability, as well assuperior writability.

SUMMARY OF THE INVENTION

The invention is a perpendicular magnetic recording medium with a RLstructure that includes an exchange-spring structure and a ferromagneticlateral coupling layer (LCL) that mediates intergranular exchangecoupling in the exchange-spring structure. The exchange-spring structureis made up of two ferromagnetically exchange-coupled magnetic layers(MAG1 and MAG2), each with perpendicular magnetic anisotropy. MAG1 andMAG2 may have a coupling layer (CL) located between them that permitstuning to the appropriate ferromagnetic inter-layer coupling strengthbetween MAG1 and MAG2. The LCL is in direct contact with MAG1 and islocated either above or below MAG1.

The LCL may be formed of Co, or ferromagnetic Co alloys, such as CoCralloys. The Co alloys may include one or both of Pt and B. Theferromagnetic alloy in the LCL has significantly greater intergranularexchange coupling than the ferromagnetic alloy in MAG1 with which it isin contact, which typically will include segregants such as the oxidesof Si, Ta, Ti, Nb, Cr, V, and B. The LCL alloy should preferably notinclude any oxides or other non-metallic segregants, which would tend toreduce intergranular exchange coupling in the LCL. Because the LCL grainboundaries overlay the boundaries of the generally segregated anddecoupled grains of MAG1 with which it is in contact, and the LCL andMAG1 grains are strongly coupled perpendicularly, the LCL introduces aneffective intergranular exchange coupling in MAG1, or more precisely itenables a combined LCL+MAG1 system with a tunable level of intergranularexchange.

The invention is also a perpendicular magnetic recording system thatincludes the above-described medium and a magnetic recording write head.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic of a prior art perpendicular magnetic recordingsystem.

FIG. 2 is a schematic of a cross-section of a prior art perpendicularmagnetic recording disk showing the write field H acting on therecording layer (RL).

FIG. 3A is a schematic of a cross-section of a perpendicular magneticrecording disk with an exchange-spring recording layer (RL) made up oftwo ferromagnetically exchange-coupled magnetic layers (MAG1 and MAG2).

FIG. 3B is a schematic of a cross-section of a perpendicular magneticrecording disk with an exchange-spring recording layer (RL) made up oftwo magnetic layers (MAG1 and MAG2) separated by a non-magnetic orweakly ferromagnetic coupling layer (CL), and the fields H1 and H2acting on MAG1 and MAG2, respectively.

FIG. 4A is a schematic showing one implementation of the inventionwherein the lateral coupling layer (LCL) is deposited directly on MAG1.

FIG. 4B is a schematic showing another implementation of the inventionwherein MAG1 is deposited directly on the LCL.

FIGS. 5A-5B are schematic illustrations of the grains and magnetizationsin MAG1 without the LCL (FIG. 5A) and with the LCL (FIG. 5B).

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3A is a schematic of a cross-section of a perpendicular magneticrecording disk according to the prior art with an exchange-springrecording layer (RL) made up of two ferromagnetically exchange-coupledmagnetic layers (MAG1 and MAG2). MAG1 and MAG2 each has perpendicularmagnetic anisotropy. However, MAG1 and MAG2 have different magneticproperties, so that they respond differently to the applied write field.For example, one of MAG1 and MAG2 can be magnetically soft and the othermagnetically hard. The magnetic grains in the hard layer areexchange-decoupled from one another, meaning that there is very lowintergranular exchange coupling in the hard layer. With a properinterlayer exchange coupling between the grains in MAG1 and MAG2, thesoft grains will rotate first under the applied write field, while atthe same time providing an exchange field to the hard grains to assistin the magnetization reversal of the grains in the hard layer, thuscausing a magnetization reversal that emulates a tilting of theeffective easy axis. MAG2, which is located closer to the write head andtypically is formed of the lower-H_(k) material, is sometimes called theexchange-spring layer (ESL), and MAG1, the lower layer and typicallyformed of the higher-H_(k) material, is sometimes called the media layer(ML).

FIG. 3B illustrates an exchange-spring medium like that described in thepreviously-cited pending application Ser. No. 11/372,295 wherein acoupling layer (CL) is located between MAG1 and MAG2. The composite RLhas at least two ferromagnetically exchange-coupled magnetic layers(MAG1 and MAG2), each with generally perpendicular magnetic anisotropyand with substantially similar anisotropy fields H_(k), that areseparated by the CL. The CL provides the appropriate ferromagneticcoupling strength between the magnetic layers. The composite RLstructure takes advantage of the depth-dependent write field H. i.e., ingeneral a write head produces a larger magnetic field and larger fieldgradient near the surface of the RL, while the field strength decreasesfurther inside the RL. The high field and field gradient near the top ofthe RL, where MAG2 is located, enables MAG2 to be formed of a high-H_(k)material. As the magnetization of MAG2 is reversed by the write field itassists in the magnetization reversal of the lower magnetic layer MAG1.In this non-coherent reversal of the magnetizations of MAG1 and MAG2,MAG2 changes its magnetization orientation in response to a write fieldand in turn amplifies the “torque,” or reverse field, exerted on MAG1,causing MAG1 to change its magnetization direction in response to aweaker write field than would otherwise be required in the absence ofMAG2. Although the write field acting on MAG1 can be significantly lessthan the write field acting on MAG2, MAG1 can have substantially thesame H_(k) because of the torque created by the magnetization reversalof MAG2. MAG1 and MAG2 can thus have substantially similar anisotropyfields Hk and could even have substantially the same materialcomposition.

The medium in the form of a disk is shown in sectional view in FIG. 3Bwith the write field H. As shown in the expanded portion of FIG. 3B, atypical grain 10 in MAG2 has a generally perpendicular or out-of-planemagnetization along an easy axis 12, and is acted upon by a write fieldH2. A typical grain 20 in MAG1 below the MAG2 grain 10 also has aperpendicular magnetization along an easy axis 22, and is acted upon bya write field H1 less than H2. In the presence of the applied writefield H2, the MAG2 acts as a write assist layer by exerting a magnetictorque onto MAG1 that assists in reversing the magnetization of MAG1.

In this invention a multilayer RL structure with an additional layer inthe exchange-spring RL has the improved writability of exchange-springRLs as well as the noise reduction and thermal stability improvementfound in RL structures that have an elevated level of intergranularexchange coupling. As shown in one implementation in FIG. 4A, anadditional layer, called a lateral coupling layer (LCL), is located ontop of and in contact with MAG1 in the exchange-spring structure. Asshown in another implementation in FIG. 4B, the LCL is located belowMAG1 with MAG1 on top of and in contact with the LCL. The LCL mediatesthe intergranular exchange coupling in the exchange-spring structure.While the LCL is depicted in FIGS. 4A and 4B as being implemented withan exchange-spring structure that includes a CL, like that shown in FIG.3B, the LCL is also fully applicable to an exchange-spring structurewithout a CL, like that shown in FIG. 3A.

A representative disk structure for the invention shown in FIGS. 4A-4Bwill now be described. The hard disk substrate may be any commerciallyavailable glass substrate, but may also be a conventional aluminum alloywith a NiP surface coating, or an alternative substrate, such assilicon, canasite or silicon-carbide.

The adhesion layer or OL for the growth of the SUL may be an AlTi alloyor a similar material with a thickness of about 2-10 nm. The SUL may beformed of magnetically permeable materials such as alloys of CoNiFe,FeCoB, CoCuFe, NiFe, FeAlSi, FeTaN, FeN, FeTaC, CoTaZr, CoFeTaZr, CoFeB,and CoZrNb. The SUL may also be a laminated or multilayered SUL formedof multiple soft magnetic films separated by nonmagnetic films, such aselectrically conductive films of Al or CoCr. The SUL may also be alaminated or multilayered SUL formed of multiple soft magnetic filmsseparated by interlayer films that mediate an antiferromagneticcoupling, such as Ru, Ir, or Cr or alloys thereof.

The EBL is located on top of the SUL. It acts to break the magneticexchange coupling between the magnetically permeable films of the SULand the RL and also serves to facilitate epitaxial growth of the RL. TheEBL may not be necessary, but if used it can be a nonmagnetic titanium(Ti) layer; a non-electrically-conducting material such as Si, Ge andSiGe alloys; a metal such as Cr, Ru, W, Zr, Nb, Mo, V and Al; a metalalloy such as amorphous CrTi and NiP; an amorphous carbon such asCN_(x), CH_(x) and C; or oxides, nitrides or carbides of an elementselected from the group consisting of Si, Al, Zr, Ti, and B. If an EBLis used, a seed layer may be used on top of the SUL before deposition ofthe EBL. For example, if Ru is used as the EBL, a 1-8 nm thick NiFe orNiW seed layer may be deposited on top of the SUL, followed by a 3-30 nmthick Ru EBL. The EBL may also be a multilayered EBL.

The MAG1 and MAG2 layers may be formed of any of the known amorphous orcrystalline materials and structures that exhibit perpendicular magneticanisotropy. Thus, the MAG1 and MAG2 may each be a layer of granularpolycrystalline cobalt alloy, such as a CoPt or CoPtCr alloy, with asuitable segregant such as oxides of Si, Ta, Ti, Nb, Cr, V and B. Also,MAG1 and MAG2 may each be composed of multilayers with perpendicularmagnetic anisotropy, such as Co/Pt, Co/Pd, Fe/Pt and Fe/Pd multilayers,containing a suitable segregant such as the materials mentioned above.In addition, perpendicular magnetic layers containing rare earthelements are useable for MAG1 and MAG2, such as CoSm, TbFe, TbFeCo, GdFealloys. MAG1 (also called the media layer or ML) and MAG2 (also calledthe exchange-spring layer or ESL) may have substantially differentmagnetic properties, such as different anisotropy fields (H_(k)), toassure that they respond differently to the applied write field andthereby exhibit the exchange-spring behavior to improve writability.MAG1 and MAG2 may also have substantially the same anisotropy fieldH_(k), meaning that the H_(k) value for the layer with the lower H_(k)is at least 70% (and up to at least 90%) of the H_(k) value for thelayer with the higher H_(k), and still exhibit the exchange-springbehavior as described above for the medium shown in FIG. 3B.

The CL may be a hexagonal-close-packed (hcp) material, which can mediatea weak ferromagnetic coupling and also provide a good template for thegrowth of MAG2. Because the CL must enable an appropriate interlayerexchange coupling strength, it should be either nonmagnetic or weaklyferromagnetic. Thus the CL may be formed of RuCo and RuCoCr alloys withlow Co content (<about 65 atomic percent), or CoCr and CoCrB alloys withhigh Cr and/or B content (Cr+B>about 30 atomic percent). Si-oxide orother oxides like oxides of Ta, Ti, Nb, Cr, V and B may be added tothese alloys. The CL may also be formed of face-centered-cubic (fcc)materials, such as Pt or Pd or alloys based on Pt or Pd, because thesematerials enable a ferromagnetic coupling between magnetic layers oftunable strength (i.e., they reduce the coupling by increasing thethickness) and are compatible with media growth.

Depending on the choice of material for CL, and more particularly on theconcentration of cobalt (Co) in the CL, the CL may have a thickness ofless than 3.0 nm, and more preferably between about 0.2 nm and 2.5 nm.Because Co is highly magnetic, a higher concentration of Co in the CLmay be offset by thickening the CL to achieve an optimal interlayerexchange coupling between MAG1 and MAG2. The interlayer exchangecoupling between MAG1 and MAG2 may be optimized, in part, by adjustingthe materials and thickness of the CL. The CL should provide a couplingstrength sufficient to have a considerable effect on the switching field(and the switching field distribution), but small enough to not couplethe MAG1 and MAG2 layers rigidly together.

The LCL may be formed of Co, or ferromagnetic Co alloys, such as CoCralloys. The Co alloys may include one or both of Pt and B. The LCL isdeposited directly on MAG1 in the FIG. 4A implementation, or the LCL isdeposited on the EBL and MAG1 is deposited directly on the LCL in theFIG. 4B implementation. The ferromagnetic alloy in the LCL hassignificantly greater intergranular exchange coupling than theferromagnetic alloy in MAG1. The LCL alloy should preferably not includeany oxides or other non-metallic segregants, which would tend to reduceintergranular exchange coupling in the LCL. Because the LCL grainboundaries overlay the boundaries of the generally segregated anddecoupled grains of the MAG1 with which it is in contact, and the LCLand MAG1 grains are strongly coupled perpendicularly, the LCL introducesan effective intergranular exchange coupling in the MAG1, or moreprecisely it enables a combined LCL+MAG1 system with a tunable level ofintergranular exchange. This is depicted in FIGS. 5A-5B, whichillustrate schematically the grains and magnetizations in MAG1 withoutthe LCL (FIG. 5A) and with the LCL (FIG. 5B). The total LCL+MAG1thickness should be in the range of approximately 2-15 nm, preferably inthe range of approximately 3-10 nm. The LCL portion of the totalLCL+MAG1 thickness should be between about 10-90%, with a preferredrange of about 20-60%. The optimal LCL thickness can be determinedexperimentally by varying the thickness and measuring the performance ofthe disks to determine which thickness provides the most suitable levelof intergranular exchange coupling for the combined LCL+MAG1 system.

The OC formed on top of the RL may be an amorphous “diamond-like” carbonfilm or other known protective overcoats, such as Si-nitride.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A perpendicular magnetic recording medium comprising: a substrate; afirst ferromagnetic layer on the substrate and having an out-of-planeeasy axis of magnetization; a second ferromagnetic layer having anout-of-plane easy axis of magnetization, the second layer beingferromagnetically exchange-coupled to the first layer; and aferromagnetic lateral coupling layer (LCL) in contact with the firstferromagnetic layer.
 2. The medium of claim 1 wherein the LCL is locatedbetween the substrate and the first ferromagnetic layer.
 3. The mediumof claim 1 wherein the LCL is located between the first ferromagneticlayer and the second ferromagnetic layer.
 4. The medium of claim 1wherein the LCL is selected from the group consisting of Co and aferromagnetic Co alloy.
 5. The medium of claim 4 wherein the LCL is aferromagnetic Co alloy comprising Cr and an element selected from thegroup consisting of B and Pt.
 6. The medium of claim 4 wherein the LCLis a ferromagnetic alloy consisting essentially of only Co and Cr. 7.The medium of claim 1 further comprising a coupling layer (CL) betweenthe first ferromagnetic layer and the second ferromagnetic layer andpermitting ferromagnetic exchange coupling of the first ferromagneticlayer with the second ferromagnetic layer.
 8. The medium of claim 7wherein the LCL is located directly on the first ferromagnetic layerbetween the first ferromagnetic layer and the second ferromagnetic layerand the CL is located between the LCL and the second ferromagneticlayer.
 9. The medium of claim 7 wherein the CL is formed of a materialselected from the group consisting of (a) a RuCo alloy with Co less thanabout 65 atomic percent, (b) a RuCoCr alloy with Co less than about 65atomic percent, and (c) an alloy of Co and one or more of Cr and B withthe combined content of Cr and B greater than about 30 atomic percent.10. The medium of claim 1 wherein each of the first and secondferromagnetic layers comprises a granular polycrystalline cobalt alloyand an oxide of one or more of Si, Ta, Ti, Nb, Cr, V and B.
 11. Themedium of claim 1 wherein the first and second ferromagnetic layers havesubstantially similar anisotropy fields.
 12. The medium of claim 1wherein the first and second ferromagnetic layers have substantiallydifferent anisotropy fields.
 13. The medium of 1 further comprising anunderlayer of magnetically permeable material on the substrate and anexchange break layer between the underlayer and the first ferromagneticlayer for preventing magnetic exchange coupling between the underlayerand the first ferromagnetic layer.
 14. A perpendicular magneticrecording disk comprising: a substrate; an underlayer of magneticallypermeable material on the substrate; an exchange-spring structure on theunderlayer and comprising first and second exchange-coupledferromagnetic layers, each of said first and second layers having anout-of-plane easy axis of magnetization and comprising a granularpolycrystalline cobalt alloy, said first ferromagnetic layer furthercomprising an oxide of one or more of Si, Ta, Ti, Nb, Cr, V and B; and aferromagnetic lateral coupling layer (LCL) in contact with the firstlayer, the LCL comprising an oxide-free ferromagnetic alloy comprisingCo and Cr.
 15. The disk of claim 14 wherein the LCL is located betweenthe substrate and the first layer.
 16. The disk of claim 14 wherein theLCL is located between the first layer and the second layer.
 17. Thedisk of claim 14 wherein the LCL alloy includes an element selected fromthe group consisting of B and Pt.
 18. The disk of claim 14 furthercomprising a coupling layer (CL) between the first ferromagnetic layerand the second ferromagnetic layer and permitting ferromagnetic exchangecoupling of the first ferromagnetic layer with the second ferromagneticlayer.
 19. The disk of claim 18 wherein the LCL is located directly onthe first ferromagnetic layer between the first ferromagnetic layer andthe second ferromagnetic layer and the CL is located between the LCL andthe second ferromagnetic layer.
 20. The disk of claim 18 wherein the CLis formed of a material selected from the group consisting of (a) a RuCoalloy with Co less than about 65 atomic percent, (b) a RuCoCr alloy withCo less than about 65 atomic percent, and (c) an alloy of Co and one ormore of Cr and B with the combined content of Cr and B greater thanabout 30 atomic percent, (d) Pt, (e) Pd, (f) Pt-based alloys, and (g)Pd-based alloys.
 21. The disk of claim 14 further comprising an exchangebreak layer between the underlayer and the first ferromagnetic layer forpreventing magnetic exchange coupling between the underlayer and thefirst ferromagnetic layer.
 22. A perpendicular magnetic recording systemcomprising: the disk of claim 14; a write head for magnetizing regionsin the second ferromagnetic layer, the ferromagneticallyexchange-coupled first ferromagnetic layer, and the LCL in contact withthe first ferromagnetic layer of said disk; and a read head fordetecting the transitions between said magnetized regions.